This invention relates to an apparatus for measuring the velocity or the rate of flow of a flowing fluid.
It is known that the velocity of a fluid flow and accordingly the rate of flow of a fluid through a conduit can be measured by disposing an elongate vortex-generating body, for example a cylindrical body, in a fluid flow and detecting the frequency of periodic generation of Karman vortices attributed to the existence of this body in the flow since the frequency is proportional to the flow velocity and, hence, to the rate of flow. A flowmeter or a flow-velocity detector on this principle is advantageous particularly in that the apparatus does not undergo changes in its velocity-detecting characteristic even with the passage of time because the relationship between the flow velocity and the vortex-generation frequency is determined solely by the thickness of the vortex-generating body (diameter in the case of a cylinder) and that the apparatus can provide a pulse signal indicative of the detected flow velocity or flow rate and accordingly facilitates digital processing of the obtained data.
However, flowmeters of this type have a disadvantage that in a pulsative flow the measurement of the flow velocity or flow rate becomes difficult primarily because stable generation of Karman vortices cannot be expected under pulsating flow conditions. In the case of using a flowmeter of this type in measuring the rate of flow of air drawn into an internal combustion engine by way of example, the accuracy of measurement cannot be guaranteed under full-throttle and nearly full-throttle conditions because of the tendency of Karman vortices being generated in synchronization with pulsation of the sucked air and/or because of a significant disturbance of the output signal by the influence of a pulsative flow of the air.
A flowmeter of the hot wire type, an example of other types of known apparatus for measuring the velocity or flow rate of fluids, comprises an electrical resistance heating wire element stretched in a fluid flow with application of a heating current thereto to detect the flow velocity by measuring the cooling effect of the flowing fluid on the heated wire element or the amount of heat the wire element is deprived of per unit time. This flowmeter is advantageous particularly in its capability of conducting measurements over a very wide range of flow velocities and invariableness of its performance even in a pulsative flow. However, this device undergoes considerable changes in its velocity-detecting characteristic with the passage of time because the heat radiation characteristic of the heated wire varies as the wire is gradually contaminated during use.